The invention relates to the field of vehicular automatic transmissions, and more particularly to a method for controlling a gear shift in an automatic transmission of a motor vehicle.
Improving the shift quality remains an important goal in the development of multi-ratio automatic transmissions. The shift quality has a significant impact on the comfort of an automatic transmission for passenger cars. Great effort is made to further raise the quality level for all modes of shifting, for example, acceleration upshift and downshift, deceleration upshift and downshift. Achieving a good shift quality is particularly important during coasting-to-stop shifts, in other words, during deceleration downshifts at low speeds, without the demand of a motor load by the driver, because the driver does not expect a noticeable reaction of the drive train. In automatic transmissions where deceleration shifting, and in particular, coasting-to-stop shifting occurs due to a simple overlapping of two friction shift elements, without a free-wheeling condition as an additional shift element, it is a known fact that the shifting sequence is difficult to implement. Due to the essentially load-free condition of the engine and the low shift pressure, corresponding to the load condition, for the engaging shift element of the respective deceleration shift, all variations as well as temporary torque and speed changes that act upon the respective shift element have a particularly strong effect on the shifting sequence, with the result of variations in the shifting quality. Examples of such variables disturbing the control over overlapping shifting includes the idle speed control of the engine, restarting of the engine after an active deceleration shut down of the engine, a changing brake gradient when the vehicle is coasting, and also different transmission oil temperatures.
The present invention provides an improved method for controlling a gear shift in an automatic transmission of a motor vehicle, especially where said gear shift is carried out as a deceleration shift, with at least one engaging shift element and without a mechanical free-wheeling condition.
This objective is accomplished by simulating a free wheeling condition by implementing a slip operation, or by an opening of a second shift element of the automatic transmission that is located in the power flow path, during the shifting sequence of the deceleration shift.
Thus, the present invention seeks to simulate a mechanical free-wheeling condition, which is not actually involved or not present with a deceleration shift, through a slip operation or an opening of an additional shift element of the automatic transmission, whereby the additional shift element is present in the flow of power of the automatic transmission independent of the shift element that engages at the deceleration shift. Through the slip operation, or the opening of the additional shift element, a drive of the automatic transmission is, partially or entirely, decoupled during the shifting sequence of the deceleration shift from a drive motor that drives the automatic transmission, such that no reaction forces or reaction factors of the deceleration shift that impact the comfort, are transferred to a drive axle of the vehicle.
Preferably, the freewheeling condition of the deceleration shift is simulated via a start-up clutch or a start-up brake of the automatic transmission, as an additional function of the start-up shift element, aside from the known function of shifting into gear, and if applicable, to decouple the engine during a vehicle stop (reduction of creep tendency). Prior to the deceleration shift, during the normal driving operation of the vehicle, the start-tip shift element is closed. At the end of the deceleration shift, the start-up element will again be closed, until the vehicle comes to a stop, and can be partially or fully opened in the case of an active stop-decoupling function when the vehicle is stopped (xe2x80x9cstandby controlxe2x80x9d). Since a sensitively operating pressure or speed control of the start-up shift element is already present, for example for the position change shifts or for the stop-decoupling function, the additional expenditure for the application of the additional function subject to the present invention is correspondingly low.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompanying drawings, in which like numerals refer to like parts.